High temperature superconductor/insulator composite thin films with Josephson coupled grains

ABSTRACT

High temperature superconductor composite thin film devices with easily moved Josephson vortices are described having high Tc and good magnetic vortex properties. A preferred composite material was YBCO/CeO2 thin film on a MgO substrate. The superconductor composites were preferably formed by off-axis co-sputtering. A surprising recovery in properties was seen after plasma etching with oxygen.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a superconductive high temperaturecomposite thin film material and a method of preparation wherein thecomposite material has interspersed superconductor and insulator grains.The superconductive composite thin film material has a predeterminedcritical current density, a high critical temperature and easily movedJosephson vortices. The critical current density is determined by therelative amounts of high temperature superconductor and insulator. Theinvention particularly relates to a composite film of a superconductorsuch as YBa₂ Cu₃ O₇ (YBCO), TlCa₂ Ba₂ Cu₃ O₈, Bi₂ (Sr,Ca)₃ Cu₂ O₈,La_(2-x) Sr_(x) CuO₄ or other rare earth barium copper oxidesuperconductors and one or more non-diffusing insulators such as ceriumoxide, (CeO₂), magnesium oxide (MgO), LaAlO₃ or SrTiO₃ deposited on asubstrate that is compatible with the high temperature superconductorand with the superconductive high temperature composite material havingeasily moved Josephson vortices.

2. Description of the Related Art

There is considerable interest in high transition temperaturesuperconducting devices and high temperature superconducting electronicsresearch focusing on the development of active three-terminal deviceshaving gain. See J. E. Nordman, Semiconductor Science and Technology, 8,681 (1995); R. Gerdemann, L. Alff, A. Beck, O. M. Froelich, B. Mayer andR. Gross, IEEE Transactions in Applied Superconductivity 5, 3292 (1995);Y. M. Zhang, D. Winkler, P. A. Nilsson, and T. Claeson, Applied PhysicsLetters, 64 1153 (1994); and K. Miyahara, S. Kubo, and M. Suzuki,Journal of Applied Physics 76, 4772 (1994), all herein incorporated byreference. The vortex flow transistor reported by J. S. Martens. G. K.G. Hohenwarter, J. B. Beyer, J. E. Nordman and D. S. Ginley, J. Appl.Phys. 65, 4057 (1989), herein incorporated by reference, the fluxonicjunction transistor proposed by A. M. Kadin, J. Appl. Physics 68,5741(1990) herein incorporated by reference and the long Josephsonjunction biased in the flux flow mode reported by D. P. McGinnis, J. E.Nordman and J. B. Beyer, IEEE Trans, Magn. MAG-23, 699 (1987), hereinincorporated by reference, are examples of devices whose gain effect isdependent on the control of magnetic vortices in the device. The firsttwo devices rely on the movement of Abrikosov vortices in a thinsuperconducting film while the third device uses the motion of Josephsonvortices along the length of a tunnel junction. The long hystereticJosephson junction vortex flow device has only been reported using lowtransition temperature (T_(c)) electrode materials. However, Gerdemannet al. have reported fabricating arrays of parallel bicrystal grainboundary junctions into high T_(c) vortex flow transistors. Single-layervortex flow devices require a region of easily moved vortices forcontrolling the transport with a small external magnetic field. Vortexmotion can be realized in a device by incorporating an array of weaklinks into the structure or by the use of a grain boundary Josephsonjunction.

The successful development of three-terminal high temperaturesuperconducting devices having gain can have a significant impact on theviability of superconducting electronics. The superconducting vortexflow transistor has been demonstrated in both low T_(c) and hightemperature superconductive materials. The superconducting vortex flowtransistor's speed and frequency response is dependent on the materialproperties and kind of vortices (Abrikosov or Josephson) responsible forvortex transport. Other superconducting flux flow and fluxonic devicesinclude those of Hohenwater et al., Characteristics of superconductingflux-flow transistors, IEEE Trans. Magn., vol. 27, pp. 3297-3300 (March1991), herein incorporated by reference. See also Kadin, Duality andfluxonics in superconducting devices, J. Appl. Phys., vol. 68, pp.5741-5749 (December 1990), herein incorporated by reference. Thesedevices are based on the motion of either Abrikosov or Josephsonvortices and require a material with properties which do notsubstantially impede the flow of magnetic flux. The high pinningstrength of YBa₂ Cu₃ O₇ (YBCO) has made it unsuitable for flux flowdevices without modifying the YBCO in some manner such as thinning ortaking advantage of naturally occurring defects such as the grainboundary junction formed over a substrate step. See Martens et al.,S-parameter measurements on single superconducting thin-filmthree-terminal devices made of high T_(c) and low T_(c) materials, J.Appl. Phys., vol. 65, pp. 4057-4060 (May 1989), herein incorporated byreference. See also Martens et al., Fluxflow microelectronics, IEEETrans. Appl. Super., vol. 3, pp. 2295-2302 (March 1993), hereinincorporated by reference.

Researchers have sought practical, three terminal, superconductingdevices for applications in hybrid technologies and on-chip integrationwith passive, superconducting components. Such devices included theflux-flow transistor and the fluxonic junction transistor, both of whichrequire a superconducting material in which vortices can easily move.

High quality high temperature superconductor (HTS) thin filmshaving"easily movable vortices" are difficult to fabricate. High qualitythin films of YBCO generally have T_(c) 's approaching 90 K (degreesKelvin) and J_(c) 's at 77 K greater than 1×10⁶ A/cm² and show strongvortex pinning. In such materials, vortex motion is difficult exceptvery close to T_(c) or in very high magnetic fields (10's of Tesla). SeeRose-Innes et al., Introduction to Superconductivity, 2nd Edition,International Series in Solid State Physics, Vol. 6, Pergamon Press, NewYork, at pp. 186-190 (1978), herein incorporated by reference.

Materials having easy vortex motion usually have a reduced T_(c) andJ_(c) and are chemically unstable in the ambient environment. This isbecause the material within or at the grain boundaries often consists ofimpurities or off-stoichiometric material, causing a reduced T_(c),J_(c) and chemical stability. For example, oxygen-deficient YBCO filmswhich have reduced T_(c) 's and J_(c) 's as well as weak pinning havebeen shown to be very susceptible to damage from device processing andexposure to water-based chemicals. See L H. Allen et al., Thin filmcomposites of Au and YBa₂ Cu₂ O-₋δ, Appl. Phys. Lett., vol. 66(8), pp.1003-1005 (20 Feb., 1995), herein incorporated by reference. Once thesematerials are fabricated into vortex flow devices, they degrade andchange their operating characteristics with age.

Even materials that were initially high quality are susceptible toprocessing damage. For example, weak-link microbridges fabricated fromhigh-quality materials have exhibited easy vortex motion. However, whenmade and used in flux flow devices, they are often operated at reducedtemperatures because the T_(c) of the microbridge is degraded by thepatterning process. See Miyahara et al., Vortex Flow Characteristics ofHigh-T_(c) Flux Flow Transistors, J. Appl. Phys., vol 75, pp. 404(1994), herein incorporated by reference. Furthermore, the stabilitywith time of these devices is uncertain because of the inherent chemicalinstability associated with degraded superconducting material.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide acomposite thin film that consists of a random array of Josephson coupledgrains in which Josephson vortices can easily move throughout thecomposite film and with the composite consisting of a random array ofsuperconducting grains separated from one another by insulating grains.

A principal object of the present invention is to provide a compositethin film that consists of a random array of Josephson coupled grains inwhich Josephson vortices can easily move throughout the composite film,where T_(c) is inversely related to the intergranular coupling strength.

The invention further comprises composite thin films that are chemicallystable and a process for making these composite thin films.

Yet another object of the present invention is to provide a compositethin film that consists of a random array of Josephson coupled grains inwhich Josephson vortices can easily move throughout the compositematerial and wherein the film "as-grown" will have the desired propertyof easy flux motion.

Still further object of the invention is to provide a composite thinfilm whose superconductive properties can be recovered by furtherprocessing steps after fabrication of a device.

It is therefore still another object of the present invention to providea composite thin film material that consists of a random array ofJosephson coupled grains in which Josephson vortices can easily movethroughout the composite film, wherein the composite film can beincorporated into a fluxonic junction diode.

It is therefore another object of the present invention to provide acomposite thin film that consists of a random array of Josephson coupledgrains in which Josephson vortices can easily move throughout thecomposite film, wherein the composite material can be incorporated intoa vortex flow transistor.

It is therefore another object of the present invention to provide acomposite thin film that consists of a random array of Josephson coupledgrains in which Josephson vortices can easily move throughout thecomposite film wherein the composite material can be incorporated into aJosephson junction transistor.

It is therefore another object of the present invention to provide acomposite thin film that consists of a random array of Josephsonjunctions in which Josephson vortices can easily move throughout thecomposite film wherein the composite film can be incorporated into abolometric device.

These and other objects are accomplished by forming a composite materialcomprising a thin film of a high temperature superconductor material ona stable substrate wherein said superconductor comprises grains of ahigh temperature superconductor separated by a non-superconductingmaterial that does not interact with the high temperaturesuperconductor.

These composite thin films can be formed by conventional techniquesincluding chemical vapor deposition, pulsed laser deposition, andoff-axis co-sputtered deposition.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention and several of theaccompanying advantages thereof will be readily obtained by reference tothe following detailed description and accompanying drawings, wherein:

FIG. 1 is a plot of the mid-point T_(c) in K versus the % CeO₂ byvolume. The figure shows the decrease in T_(c) as the volume of CeO₂increases. The bars show ΔT_(c), the width of the resistive transitionfor each value of CeO₂.

FIG. 2 is a plot of the magnetic field dependence of I_(c) at a reducedtemperature of 0.9 for a 25.9% by volume CeO₂ bridge.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention is provided to aidthose skilled in the art in practicing the invention. However, thefollowing detailed description of the invention should not be construedto unduly limit the present invention which is limited by the claims andapplicable law. Variation and modification in the embodiments describedbelow may be made by those of ordinary skill in the art withoutdeparting from the scope of the present invention.

The term "composite" is used herein to describe a thin film materialsystem comprising at least two distinct materials interspersed withineach other forming a two-dimensional array of the two different kinds ofmaterials. The term "composite" additionally describes a thin filmmaterial system wherein a grain of one material is coated orencapsulated by a different material so as to form an inclusion ofnon-superconductive material along the grain boundaries of thesuperconductor.

The term "device" is used herein to describe an element of a circuit,for example a vortex flow transistor, fluxonic junction diode, Josephsonjunction transistor, capacitors, resistors and interconnecting lines toform a circuit. These circuits can then be patterned intosuperconducting chips as is well known in the art.

With respect to superconducting materials, the T_(c) is the criticaltemperature of a material below which temperature the superconductingmaterial exhibits no measurable voltage during current flow. Inaddition, a superconducting material also has a J_(c) which is thecritical current density. If a current is passed through a sample ofsuperconducting material and if the current is above I_(c) (the criticalcurrent), then the superconducting sample exhibits a measurable voltagedue to current flow. The value of J_(c) is proportional to the value ofI_(c) (J_(c) =I_(c) /cross-sectional area of current flow). The lowerthe value of J_(c) the lower the value of I_(c) and, therefore, thelower the threshold current that is required for voltage development,(vortex movement).

The purpose of this invention is the fabrication of thin film hightemperature superconductors (HTS) with the important material propertyof "easily moved" magnetic vortices. This property may be associatedwith material systems having a reduced critical current densityresulting from reduced intergranular coupling. For example, compositethin film material systems of a high temperature superconductor such asYBCO and a non-diffusing insulator such as CeO₂ exhibit this property.For low CeO₂ concentrations the superconducting onset temperature is notsignificantly reduced in these composite material films, suggesting thatthe CeO₂ insulator is segregated into the grain boundaries and thisforms a Josephson-junction-like structure wherein the superconductinggrains are coupled. This intergranular coupling can be either SNS orSIS, wherein S is a superconductor, N is a normal metal and I is aninsulator. In the examples given herein the CeO₂ is the "N" or "I"portion of the junction and the YBCO is the "S" portion of the junction.

The thin film composite materials can be fabricated into devices usingconventional fabrication techniques including photolithography followedby chemical etching, ion milling, laser ablation and ion beamtechniques. These composite material films are usable as materials forfabricating three terminal HTS devices which rely on vortex interactionsfor the control of electron transport. These composite material filmsalso have application as non-linear superconducting devices, bolometers,and non-bolometric photodetectors.

The high temperature superconductors are paired with insulators thatwill not diffuse into the lattice of the superconductor and destroy thesuperconducting properties thereby lowering the T_(c). Examples ofsuperconductor/insulator pairs include YBa₂ Cu₃ O₇ /CeO₂ ; YBa₂ Cu₃ O₇/MgO; Y Ba₂ Cu₃ O₇ /LaAlO₃ ; YBa₂ Cu₃ O₇ /SrTiO₃ ; Bi₂ (Ca,Sr)₃ Cu₂ O₈/CeO₂ ; Bi₂ (Ca,Sr)₃ Cu₂ O₈ /MgO; Bi₂ (Ca,Sr)₃ Cu₂ O₈ /LaAlO₃ ; Bi₂(Ca,Sr)₃ Cu₂ O₈ /SrTiO₃ ; La_(2-x) Sr_(x) CuO₄ /CeO₂ ; La_(2-x) Sr_(x)CuO₄ /MgO; La_(2-x) Sr_(x) CuO₄ /LaAlO₃ ; La_(2-x) Sr_(x) CuO₄ /SrTiO₃ ;TlCa₂ Ba₂ Cu₃ O₉ /CeO₂ ; TlCa₂ Ba₂ Cu₃ O₉ /MgO; TlCa₂ Ba₂ Cu₃ O₉ /LaAlO₃and TlCa₂ Ba₂ Cu₃ O₉ /SrTiO₃. When YBCO or other rare earth bariumcopper oxide high temperature superconductors having the formula ABa₂Cu₃ O₇₋Δ wherein Δ is between 0 and about 0.5 and A is selected from therare earth materials generally recognized as superconducting includinglutetium (Lu), Neodinium (Nd), Samarium (Sm), Europium (Eu), Gadolinium(Gd), Erbium (Er), Holmium (Ho), and Ytterbium (Yb). When the ABi₂ Cu₃O₇₋Δ and YBCO are used these compounds are in the superconductingorthorombic crystal form. See Quadri et al. U.S. Pat. No. 5,106,829herein incorporated by reference.

Preferred insulators are single metal oxides such as MgO and CeO₂because the single metal oxides are easier to maintain stoichiometryduring deposition. A problem with a mixed metal oxide such as SrTiO₃ andLaAlO₃ is that it is difficult to maintain the mixed oxide stoichiometryas the material is deposited.

The substrate can be any conventional substrate material suitable forhigh temperature superconductive film growth and includes SrTiO₃ (STO),MgO, SrLaAlO₄, SrLaGaO₄, YSZ (yttria stabilized ZrO₂), LaAlO₃, NdGaO₃,and both Si and Al₂ O₃ provided that Si and Al₂ O₃ have a buffer layerto prevent diffusion of the substrate into the high temperaturesuperconductor which would destroy the properties of the hightemperature superconductor.

A preferred high temperature superconductor/ insulator composition isYBCO/CeO₂. YBCO is preferred as a high temperature superconductorbecause of its good superconducting properties, morphology and ease offilm synthesis, and CeO₂ is preferred as the insulator because it isimmiscible with YBCO and a single metal oxide is easier to maintainstoichiometry during deposition. The concentration of CeO₂ in thecomposite thin film will determine the T_(c) and J_(c) in the compositematerial and in general concentrations above about 43% by volume CeO₂ inthe film are insulating. Therefor one will want up to about 43% byvolume CeO₂ in the film and a preferable concentration is up to about30% by volume CeO₂ in the film:

The high temperature superconductor/insulator pairs can be readilyformed by a variety of different methods. Methods for the deposition ofthe superconductor include sputtering, evaporation, laser ablation, andchemical vapor deposition. Sputtering may be conventional or off-axissputtering. Evaporation may be thermal evaporation or electron beamevaporation. Chemical vapor deposition may be metalorganic chemicalvapor deposition (MOCVD). Other methods of co-depositing a hightemperature superconductor and insulator are known in the art and anyin-situ deposition techniques are acceptable for the fabrication of thecomposite film. As previously stated, the substrate includes SrTiO₃(STO), MgO, SrLaAlO₄, SrLaGaO₄, YSZ (yttria stabilized ZrO₂) LaAlO₄,NdGaO₃ and both Si and Al₂ O₃ with buffer layers. Deposition of thesuperconductor and insulator is carried out at an appropriatetemperature wherein the substrate, superconductor and insulator arestable at the deposition temperature and where the superconductor andinsulator form a composite materials system.

A preferred deposition technique is off-axis magnetron sputtering usinga three gun off-axis sputtering chamber as described in YBa₂ CuO-_(-x)and LaAlO₃ Composite Thin Films by Off-Axis Magnetron Sputtering, Appl.Phys. Lett. 60 (3) pp 384-391 (1992) herein incorporated by reference.

The plasma etch step is very important in recovering the hightemperature superconducting properties and Josephson coupled grainproperties of the composite material. Often a composite with a highpercentage of insulator will be non-superconducting but when treatedwith oxygen the material will become superconductive. In addition thecomposite will become non-superconductive or the T_(c) of the compositewill decrease after the composite is formed into a device. When thecomposite device is subjected to oxygen plasma etch the T_(c) isrestored and the Josephson coupled grain properties are improved. Theplasma oxygen pressure can be raised to about 5 mm of Hg and a preferredplasma oxygen pressure is from about 0.1 mm Hg to about 5 mm Hg.

The invention having been generally described, the following example isgiven as particular embodiments of the invention to demonstrate thepractice and advantages thereof. It is understood that the example isgiven by way of illustration and is not intended to limit in any mannerthe specification or the claims that follow.

YBa₂ CuO₇ /CeO₂ composites have been made by co-sputtering YBCO and CeO₂onto a (100) MgO substrate. These YBCO/CeO₂ composites were described inThe Properties of Co-sputtered YBa₂ Cu₂ O- and CeO₂ Thin Films, E. J.Cukauskas and L. H. Allen, J. Appl. Phys. 80 (10), 15 Nov., 1996, pp5843-5849 herein incorporated by reference and Off-Axis Co-SputteredYBCO and CeO₂ Thin Film, E. J. Cukauskas et al., IEEE Transactions onApplied Superconductivity, vol. 7 June 1997 pp 1654-1657, hereinincorporated by reference. The YBCO/CeO₂ composites were generally madeby depositing the YBCO/CeO₂ onto a (100) MgO substrate using amulti-target off-axis sputtering system. The face to face YBCO targetswere presputtered at 0.45 A dc and the CeO₂ target was presputtered atthe appropriate rf power level for the desired composition. Compositionwas calculated from the YBCO and CeO₂ deposition rate and time. The YBCOand CeO₂ were co-sputtered for about 4 hours which resulted in a filmthickness ranging from about 1600 to 2800 Å. The resulting film wascooled to ambient temperature in one atmosphere of O₂. Upon removal fromthe sputtering chamber, silver contact pads were thermally evaporatedthrough a metal shadow mask prior to sample characterization. A seriesof YBCO/CeO₂ films were co-sputtered using these conditions, each at adifferent CeO₂ rf power level. This yielded films having a CeO₂concentration from 0% to 43% by volume for rf power levels up to 150 W.Films made with CeO₂ concentrations above about 43% were insulating.

The as-sputtered samples (with silver pads added) were characterized forroom-temperature resistance (R₃₀₀ K), T_(c), transition width (ΔT_(c)),and resistance ratio (RR). T_(c) was defined as the temperature at whichthe sample resistance became less than 1 mΩ, ΔT_(c) as the temperatureinterval between the 10 and 90% resistance points just below thetransition onset, and RR as the ratio of the resistance at 295K and100K. The T_(c) of the as-sputtered films decreased and the transitionwidth increased as more CeO₂ was incorporated into the films as can bereadily seen in FIG. 1. These observations are consistent with a randomde-coupling of the superconducting grains as the CeO₂ is increased,producing a range of T_(c) that results in a broadened transition.As-sputtered films with more than about 23% CeO₂ by volume werenon-superconducting, but surprisingly, some of these samples becamesuperconducting when subjected to an oxygen plasma etch. The oxygenplasma etch process will be discussed in more detail below. Table Isummarizes the properties of the as-sputtered films. T_(c) rapidly fallsoff above approximately 13% CeO₂, and R₃₀₀ K rises sharply at about 30%CeO₂. After characterization, the films were then scribed into a bridgegeometry for critical current measurements and magnetic field responsestudies. With the methods described above one can form YBCO/CeO₂composites with up to about 43% by volume CeO₂. Preferably the hightemperature superconductor contains up to about 30% by volume CeO₂.Superconductor composites were made that had dI_(c) /dB values greaterthan about a 2% change in critical current per gauss, a value that issuitable for forming three terminal flux flow devices.

Standard photolithography and wet chemical etching were initially usedto define bridges in some of the first YBCO/CeO₂ samples. A substantialdegradation in T_(c) and an increase in resistance was found after thisprocessing and some samples became non-superconducting after theprocessing. To avoid this degradation, samples were patterned bymechanically scribing the films into bridges one or two mm wide. Thebridge length of these samples was uncertain, and no attempt was made toestimate it. These samples were intended for investigating thetemperature dependence of the critical current (I_(c)) near T_(c).

To study further the film dependence on processing, a 15% CeO₂ samplewas patterned with photoresist and ion milled into a 1-mm-wide bridgeand afterwards subjected to an oxygen plasma etch for 20 min in a barrelplasma etcher. It was surprisingly learned that T_(c) measurements afterthe oxygen plasma etch showed a significant improvement in T_(c) and areduction in resistance over the values measured prior to the etch. Theas-sputtered film T_(c) was 68.3K, and after ion milling had degraded to51.3K. However, after plasma etching the bridge in oxygen, T_(c)increased to 75.0° K and R₃₀₀ K changed from 198 to 60.7 ω. In view ofthese results, all the scribed samples were routinely subjected to theoxygen plasma etch process before I_(c) measurements. By usingconventional photolithography, conventional patterning methods such asion milling and wet chemistry, and rejuvenating the films with an oxygenplasma etch, it is now possible to fabricate these high temperaturesuperconductor composite thin films into devices.

The oxygen plasma etching was performed on all the samples after bridgepatterning and on those as-sputtered films which were notsuperconducting. A barrel plasma etcher, model #PMO 123, manufactured byTegal Corp., Novato, Calif. was used for the treatment. The plasma etchwas performed at a pressure of 1.2 mm of oxygen and rf power level setat 40% of maximum. Samples were etched for 20 minutes following a 30minute system break-in period. However oxygen plasma pressures up toabout 5 mm Hg can be used with a preferred range of oxygen pressuresfrom about 0.1 mm to about 5 mm Hg. New contact pads had to beevaporated because the silver became discontinuous after the oxygenplasma etch. SEM micrographs showed that the silver formed an array ofspiral-shaped columnar grains several thousand Å in diameter. Followingthe treatment, the films maintained their improved properties for atleast several months.

                                      TABLE 1    __________________________________________________________________________    Summary of the properties of the as-sputtered (before) and oxygen plasma    etched (after)    YBCO/CeO.sub.2 films and bridges.    % .sub.vol             R.sub.300° K.                            T.sub.c                                 T.sub.c                                      ΔT.sub.c                                           ΔT.sub.c    CeO.sub.2        rf (W)             (Ω)                  RR.sub.before                       RR.sub.after                            (K).sub.before                                 (K).sub.after                                      (K).sub.before                                           (K).sub.after    __________________________________________________________________________    0   0    3.48 2.5  2.7  84.4 84.5 1.2  1.4    7.4 16   4.20 2.5  2.7  80.8 84.4 1.6  1.3    13.0        30   5.57 2.0  2.2  77.7 82.3 3.4  2.8    14.9        35   9.37 1.8  2.0  68.2 81.8 8.2  2.9    20.0        50   13.1 1.6  1.8  59.2 75.0 16.0 5.7    21.6        55   19.1 1.2  1.8  --   66.8 --   10.5    23.1        60   15.9 1.4  1.5  22.3 51.1 39.5 28.6    25.9        70   32.7 1.1  1.4  --   35.4 --   29.5    33.3        100  71.6 0.84 1.2  --   22.8 --   33.8    34.4        105  365  --   1.1  --   16.5 --   19.1    35.5        110  1.83k                  --   0.81 --   --   --   --    38.5        125  163k --   0.71 --   --   --   --    42.9        150  3.4M --   --   --   --   --   --    __________________________________________________________________________

The oxygen plasma etched samples were characterized by T_(c), ΔT_(c),RR, R₃₀₀ K, I_(c) (T) near T_(c), and response to small applied magneticfields. I_(c) was taken as that value of current (˜0.1 μV) which showeda clear departure from the voltage noise fluctuations. The bridges werescribed to approximate dimensions. Lengths were not precisely measuredso that an electric field criterion was not used to define I_(c).Significant improvements in the superconducting properties after theoxygen plasma treatment were seen.

A series of I_(c) measurements were taken for each sample at fixedtemperatures for several values of the applied magnetic field, one ofwhich is shown in FIG. 2. If the field response is a result of Josephsonvortices, then the response should be greater at lower fields where thesin (x)/x function of I_(c) (B) in a junction varies the most, as isshown by the data of FIG. 2. The sensitivity of I_(c) for small fieldsof several Gauss was also studied. The observed reduction of I_(c)measured as a function of small magnetic fields supports transport byJosephson vortex motion.

Detailed Synthesis of the YBCO/CeO₂ Films

The YBCO/CeO₂ films were co-sputtered onto (100) MgO substrates using amulti-target off-axis sputtering chamber. The details of the chamberhave been described in YBa₂ CuO-_(-x) and LaAlO₃ Composite Thin Films byOff-Axis Magnetron Sputtering, Appl. Phys. Lett. 60 (3) pp 384-391(1992) herein incorporated by reference. The MgO substrates werepolished on one side and came from the manufacturer coated with mineraloil to keep out moisture. The substrates were thoroughly cleaned intrichlorethane, methanol and isopropanol after which they were subjectedto a 40 h bake at 1000° C. in flowing oxygen. The extended bake wasundertaken to assure all trace hydrocarbons and water contamination wereremoved. The substrates were pasted onto a stainless steel substrateholder using "Leitsilber 200" silver paint from Ted Pella, Inc., P.O.Box 492477 Redding Calif. 96049. They were then baked on a hot plate at120° C. for one hour prior to loading into the vacuum chamber and leftin one atmosphere of oxygen until ready for film growth. The depositionsequence began with the chamber evacuation and substrate holder heatingto the deposition temperature of 680° C. in the flowing depositiongases. The sputter gas flow rates were set to 54 sccm (standard cubiccentimeters per minute) for argon, 34 sccm for oxygen, and 11 sccm forhydrogen. After 10 minutes of gas flow, the turbomolecular pump wasthrottled back and the chamber pressure set to 150 μm for filmdeposition. At this point, the targets were pre-sputtered for 30 minutesat 0.45 A dc for the face-to-face YBCO targets and the CeO₂ target atthe appropriate rf power level for the desired composition. Compositionwas calculated from YBCO and CeO₂ deposition rates and time. The shutterwas then opened and the YBCO and CeO₂ were co-sputtered for 4 hourswhich resulted in a film thickness ranging from approximately 1600 to2800 Å. The resulting film was cooled to ambient temperature in oneatmosphere of oxygen. Upon removal from the sputtering chamber, silvercontact pads were thermally evaporated through a shadow metal mask priorto sample characterization. A series of YBCO/CeO₂ films was co-sputteredusing these conditions each at a different CeO₂ rf power level. Thisyielded films having CeO₂ concentrations from 0% to 43% by volume for rfpower levels up to 150 W.

Although certain presently preferred embodiments of the presentinvention have been specifically described herein, it will be apparentto those skilled in the art to which the invention pertains thatvariations and modifications of the various embodiments shown anddescribed herein may be made without departing from the spirit and scopeof the invention. Accordingly, it is intended that the invention belimited only to the extent required by the appended claims and theapplicable rules of law.

What is claimed is:
 1. A high temperature superconductor composite thinfilm device having easy vortex flow, said device being formed from theprocess comprisingselecting a high temperature superconductor; selectingan insulator; selecting a substrate that is compatible with said hightemperature superconductor; co-depositing the high temperaturesuperconductor and insulator on the substrate thereby forming a hightemperature superconductor composite thin film with Josephson coupledgrains; forming a device in the high temperature superconductorcomposite thin film; and oxygen plasma etching the high temperaturesuperconducting composite thin film device.
 2. The high temperaturesuperconductor composite thin film device of claim 1 wherein the hightemperature superconductor is selected from YBa₂ Cu₃ O₇ ; Bi₂ (Ca,Sr)Cu₂O₇ ; La_(2-x),Sr_(x) CuO₄ ; TlCa₂ Ba₂ Cu₃ O₉ and A Ba₂ Cu₃ O₇ wherein Ais selected from lutetium, neodinium, samarium, europium, gadolinium,erbium, holmium, and ytterbium.
 3. The high temperature superconductorcomposite thin film device of claim 1 wherein the insulator is selectedfrom metal oxides that are immiscible with the high temperaturesuperconductor.
 4. The high temperature superconductor composite thinfilm device of claim 1 wherein the substrate is selected from SrTiO₃,MgO, SrLaAlO₄, SrLaGaO₄, yttria stabilized ZrO₂, LaAlO₃, NdGaO₃ andbuffer layered Si and Al₂ O₃.
 5. The high temperature superconductorcomposite thin film device of claim 1 wherein said high temperaturesuperconductor composite has greater than about 2% change in criticalcurrent per gauss.
 6. The high temperature superconductor composite thinfilm device of claim 1 wherein said device is selected from fluxonicjunction diodes, vortex flow transistors, Josephson effect transistorsor bolometers.
 7. The high temperature superconductor composite thinfilm device of claim 2 wherein said high temperature superconductor isYBa₂ Cu₃ O₇.
 8. The high temperature superconductor composite thin filmdevice of claim 3 wherein said insulator is selected from CeO₂, MgO,LaAlO₃ and SrTiO₃.
 9. The high temperature superconductor composite thinfilm device of claim 8 wherein said insulator is CeO₂.
 10. The hightemperature superconductor composite thin film device of claim 1 whereinsaid oxygen treatment is a high pressure oxygen plasma etch.
 11. A hightemperature superconductor composite thin film device having easy vortexflow, said device being formed from the process comprising:selecting asubstrate that is compatible with YBa₂ Cu₃ O₇ ; codepositing YBa₂ Cu₃ O₇and CeO₂ on said substrate to form a high temperature superconductingcomposite thin film with Josephson coupled grains; forming a device inthe high temperature superconducting composite thin film; and plasmaetching the high temperature superconducting composite thin film device.12. The high temperature superconducting composite thin film device ofclaim 11 wherein said device is selected from fluxonic junction diodes,vortex flow transistors Josephson junction transistors and bolometers.13. The high temperature superconducting composite thin film device ofclaim 11 wherein said substrate is selected from SrTiO₃, MgO, SrLaAlO₄,SrLaGaO₄, yttria stabilized ZrO₂, LaAlO₃, NdGaO₃ and buffered layer Siand Al₂ O₃.
 14. The high temperature superconducting thin film device ofclaim 11 wherein said oxygen treating is by high pressure oxygen plasmaetch.
 15. The high temperature superconducting thin film device of claim11 wherein said high temperature superconductor composite thin film hasgreater than about 2% change in critical current per gauss.
 16. The hightemperature superconducting thin film device of claim 11 wherein thehigh temperature superconductor composite thin film contains up to about43% by volume CeO₂.
 17. The high temperature superconductor thin filmdevice of claim 11 wherein said high temperature superconductorcomposite thin film contains up to about 30% by volume CeO₂.
 18. Thehigh temperature superconductor thin film device of claim 2 wherein saidhigh temperature superconductor composite thin film has a T_(c) fromabout 84.4K to about 16.5K.
 19. The high temperature superconductor thinfilm device of claim 10 wherein the oxygen plasma has a pressure up toabout 5 mm of Hg.
 20. The high temperature superconductor composite thinfilm device of claim 19 wherein the oxygen plasma has a pressure fromabout 0.1 mm up to about 5 mm of Hg.
 21. The high temperaturesuperconductor composite thin film device of claim 14 wherein the oxygenplasma has a pressure up to about 5 mm of Hg.
 22. The high temperaturesuperconductor composite thin film device of claim 2 wherein the oxygenplasma has a pressure from about 0.1 mm up to about 5 mm of Hg.